How to Choose a Step-Up or Step-Down Ring
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You've found the filter you want, and it's the wrong size for your lens. An adapter ring fixes that for a few dollars — as long as you buy the right one, in the right direction.
Short answer
Adapter rings are written as two numbers, and the order is always the same: lens size first, filter size second. A 49–52 ring screws onto a 49mm lens and gives you a 52mm filter thread.
Going up — small lens, bigger filter — is the safe direction, and it's what almost everyone wants.
Going down puts a narrower filter in front of a wider lens, which can darken the corners of your photographs. Sometimes it's fine. Often it isn't.
If you don't know your lens's filter size yet, start with how to find the filter size of any vintage lens.
What an adapter ring actually is
It's a thin ring with a male thread on one side and a female thread on the other, in two different sizes. It screws into the front of your lens, and your filter screws into it. That's the whole idea.
Most are machined from aluminium or brass. Some are moulded plastic — lighter and cheaper, but the threads wear faster and the ring can crack or deform if it's over-tightened or dropped. Either works; metal is the safer buy if you plan to move it between lenses often.
They're one of the cheapest accessories in photography, and they solve a problem that comes up constantly with vintage glass: old lenses use filter sizes that modern filters are rarely sold in.
Reading the numbers — the part people get backwards
A ring marked 49–52 means:
- 49 — the side that screws into your lens
- 52 — the size of filter it accepts
Lens first, filter second. Every time.
This is the single most common ordering mistake, because it's easy to read "49–52" as "converts 52 to 49." It doesn't. If you have a 52mm lens and a 49mm filter, the ring you want is 52–49, not 49–52.
One way to remember it: the numbers run in the order the light doesn't — from the lens outward to the filter.
Up is safe, down is a compromise
Stepping up means putting a wider filter in front of a narrower lens. Optically this costs you nothing. The filter is bigger than the lens's field of view, so it simply isn't in the way.
Stepping down means the opposite: a narrower filter in front of a wider lens. Now the filter's rim sits inside the light path, and it can appear as darkened corners in your images — vignetting.
Whether it actually shows up depends on the lens. A wide-angle lens will show it badly. A longer lens might not show it at all, especially stopped down. It's not automatically unusable, but it's a gamble, and it's worth testing before an important shoot rather than after.
If you have the choice, go up.
How to pick your target size
Here's the trick that experienced film shooters use: instead of buying a filter for every lens, pick one filter size and adapt everything to it.
Choose a size that is:
- Larger than your largest lens thread — so every ring steps up, never down
- Common — 52mm, 55mm, 58mm and 77mm are easy to find in every filter type
Then buy one good filter in that size, plus a cheap ring for each lens. One polariser instead of six. It's usually the difference between a drawer full of filters and a small pouch that covers your whole kit.
What you give up
Adapter rings are close to free optically, but they do cost you three small things. Worth knowing before you commit:
- Your front cap may no longer fit. With a ring fitted, the original cap is now the wrong size. You'll want a cap in the new size instead.
- Your lens hood may no longer fit. Hoods thread into the same filter ring, so a ring can block them — or change how far forward the hood sits.
- The lens gets slightly longer at the front. Usually a millimetre or two. It matters only on very wide lenses, where even that can cause vignetting.
One thing not to do: stacking several rings to reach a size. Each one adds length and another edge into the light path. Buy the single ring that goes straight from your lens size to your filter size.
Clamp-on rings, for lenses with no thread
Some older lenses have no filter thread at all. For those there are clamp-on (also called slip-on or push-on) adapters, which grip the outside of the front ring and give you a normal filter thread on the front.
These are sized by the outside diameter of the lens's front ring, not by any thread — so measure that instead. A clamp-on marked 50mm fits a lens whose front ring is 50mm across the outside, which is a different measurement entirely from a 50mm screw thread. Don't mix the two numbers up.
Buying used rings
Adapter rings are simple parts, and metal ones in particular survive decades with no problem. Four things are worth a look before you buy:
- The threads. Cross-threading — where someone forced it on at an angle — leaves flattened or chewed thread. That ring will fight you forever.
- Roundness. A dropped ring can go slightly oval. It'll bind partway in.
- Metal or plastic. Plastic rings are fine in use, but check them more carefully: look for hairline cracks around the thread and any sign the ring has been squeezed out of shape. Worn plastic threads are also harder to spot than worn metal ones.
- Finish inside. A matte black inner edge is better than a shiny bare one, which can bounce light back into the lens.
Beyond that, brand matters far less here than it does for glass — a ring is a spacer, not an optic.
Where to look next
And if you still need to work out what size your lens takes, that's covered here: how to find the filter size of any vintage lens.